Expert: obtains the ordinal of the field value from the default Lucene
FieldCache using getStringIndex()
and reverses the order.

The native lucene index order is used to assign an ordinal value for each field value.

Field values (terms) are lexicographically ordered by unicode value, and numbered starting at 1.
Example of reverse ordinal (rord):
If there were only three field values: "apple","banana","pear"
then rord("apple")=3, rord("banana")=2, ord("pear")=1

WARNING:
rord() depends on the position in an index and can thus change
when other documents are inserted or deleted,
or if a MultiSearcher is used.

WARNING: This API is experimental and might change in incompatible ways in the next release.

NOTE: with the switch in 2.9 to segment-based
searching, if getValues(org.apache.lucene.index.IndexReader) is invoked with a
composite (multi-segment) reader, this can easily cause
double RAM usage for the values in the FieldCache. It's
best to switch your application to pass only atomic
(single segment) readers to this API.